During storage, red blood cells (RBCs) for transfusion purposes suffer progressive deterioration. Sialylated glycoproteins of the RBC membrane are responsible for a negatively charged surface which creates a repulsive electrical zeta potential. These charges help prevent the interaction between RBCs and other cells, and especially among each RBCs. Reports in the literature have stated that RBCs sialylated glycoproteins can be sensitive to enzymes released by leukocyte degranulation. Thus, the aim of this study was, by using an optical tweezers as a biomedical tool, to measure the zeta potential in standard RBCs units and in leukocyte reduced RBC units (collected in CPD-SAGM) during storage. Optical tweezers is a sensitive tool that uses light for measuring cell biophysical properties which are important for clinical and research purposes. This is the first study to analyze RBCs membrane charges during storage. In addition, we herein also measured the elasticity of RBCs also collected in CPD-SAGM. In conclusion, the zeta potential decreased 42% and cells were 134% less deformable at the end of storage. The zeta potential from leukodepleted units had a similar profile when compared to units stored without leukoreduction, indicating that leukocyte lyses were not responsible for the zeta potential decay. Flow cytometry measurements of reactive oxygen species suggested that this decay is due to membrane oxidative damages. These results show that measurements of zeta potentials provide new insights about RBCs storage lesion for transfusion purposes.
Citation: Silva DCN, Jovino CN, Silva CAL, Fernandes HP, Filho MM, Lucena SC, et al. (2012) Optical Tweezers as a New Biomedical Tool to Measure Zeta Potential of Stored Red Blood Cells. PLoS ONE 7(2): e31778. doi:10.1371/journal.pone.0031778
Editor: Vladimir N. Uversky, University of South Florida College of Medicine, United States of America
Received: November 17, 2011; Accepted: January 18, 2012; Published: February 21, 2012
Copyright: © 2012 Silva et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: The authors are grateful to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Supeior), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), and FACEPE (Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco). AF received an award in 2008 given by L'ORÉAL/UNESCO for young women in Science in partnership with Brazilian Academy of Sciences. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors declare the following competing interest: AF received an award in 2008 given by L'ORÉAL/UNESCO/Brazilian Academy of Sciences. There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials, as detailed online in the guide for authors.
RBCs have sialylated glycoproteins which are responsible for a negatively charged membrane cell surface . In an electrolyte medium, such as in blood plasma, this induces the formation of a layer of surrounding medium ions of opposite charges rigidly bound around the cells, that creates a repulsive electrical zeta potential (ζ) between the RBCs. The ζ potential is an important property responsible to stabilize the RBCs colloidal suspension preventing cells to come too close and avoiding interactions between RBCs and other cells, and especially among themselves. In this way, the zeta potential not only regulates the adhesions among RBCs, but also between RBCs and endothelial cells, like cells from capillary walls –. Some authors have even reported a loss of sialic acid in mature cells and describe this loss as capable to decrease RBCs survival in circulation in animal models .
RBCs storage lesions can reduce post-transfusion RBCs survival in blood circulation . To determine the reasons responsible for this, a number of investigators have examined changes in several biophysical and biochemical properties during storage. It is known that one of the most common already observed storage injuries is the loss of cell elasticity or deformability . However, questions not answered yet are: RBCs membrane charges, consequently the ζ potential, can also change with storage time as well as the elasticity changes? If there are changes in ζ potential, what could be the cause? Elasticity and ζ potential can be somehow connected by some common factor? RBCs glycoproteins are sensitive to enzymes and can be even removed by them . During RBCs storage period, the leukocyte degranulation promotes enzyme release, which can reduce glycoprotein membrane expressions and probably can change the membrane electrical charges . This raises the question if the leukocyte enzymes are capable to change the ζ potential. The aim of this study was to search for answers on these issues by measuring the ζ potential of standard RBCs units and also of pre-storage leukocyte reduced RBCs units by using an optical tweezers (both samples were collected in CPD-SAGM and analyzed as a function of the storage time). We also measured RBCs elasticity, with an optical tweezers, and analyzed, by flow cytometry, the production of reactive oxygen species in standard stored RBCs units collected in the same preservative solution to correlate deformability with the ζ potential results.
Optical tweezers, a highly sensitive tool based on photon momentum transfer that uses an infrared laser beam tightly focused by the microscope objective , belong to the modern laser techniques that have shown a great contribution to optical microscopy and life sciences. Optical tweezers allow individual trapping and manipulation of biological systems, and can be used to obtain important properties of cells and molecules –. In particular, optical tweezers allow mechanical measurements of RBCs properties (such as the membrane viscosity and elasticity) of normal cells and also of cells altered by some external factors (for example: storage, ionizing radiation, action of a drug or even by hematological diseases) –. In addition, optical tweezers have recently been used to evaluate electrical properties of the membrane through measurements of zeta potential . For single-cell manipulation, another modern and powerful tool, also based on light, which can be applied to evaluate cell deformability, is the optical stretcher. Recently, J. Guck and collaborators measured the elasticity of normal and malaria infected RBCs with an optical stretcher and showed that the evaluation of cell elasticity can be used to detect early stages of malaria infection with high sensitivity and speed –.
In this paper, we show that ζ potential is an important property, as sensitive as elasticity to storage timing and conditions, which can provide new information about storage lesions for transfusion purposes. To our knowledge, this is the first study that monitors RBC membrane electrical charge changes with storage and the first time that the elasticity of RBCs collected in CPD-SAGM is analyzed and correlated to ζ potential. We quantified important RBCs parameters that can help in the comprehension of the effects caused in these cells by different blood storage conditions. We believe that the analysis of the changes of the electrical properties of RBCs membranes can also provide a better comprehension about cell senescence process and as well about hemagglutination reactions.
Materials and Methods
The optical tweezers system consists of a laser beam in the near infrared (ζ = 1064 nm – IPG Photonics, EUA) focused on the microscope (Axiolab, Carl Zeiss, Germany) through an objective of 100×, NA = 1.25. The microscope is equipped with a motorized stage (Prior Scientific, UK) and with a real time image capture system integrated to a computer.
For all applications, RBC units were obtained from Foundation of Hematology and Hemotherapy of Pernambuco (Fundação Hemope – the Ethical Committee of this Institution approved this study). All RBC units were collected in CPD-SAGM bags (Fresenius Kabi®) and stored at 4°C (±2°C). Pre storage leukocyte reduction was performed using bags with in line filters (Composelect - Fresenius Kabi®). For the zeta potential and elasticity analysis, all RBCs samples were diluted in AB serum (1∶1000 µL). Measurements of ζ were performed on the first day of each week during 36 days and at least 40 cells originated from 4 different donors were analyzed during each week. For elasticity measurements, at least 20 cells were analyzed during each week. These analyses started on day 8 and were performed until the 36th day of storage. For RBC leukodepleted samples, ζ was measured on the first day of each week during 15 days and at least 20 cells from RBC leukodepleted units were analyzed. The production of reactive oxygen species (ROS) was also quantified for standard RBCs units during the same storage period (from day 1 to day 36). For this, 106 cells/mL were washed and resuspended in PBS (Phosphate Buffered Saline), incubated for 30 min with 0.5 µL DCHF-DA (dichlorodihydrofluorescein diacetate – Invitrogen) and analyzed by flow cytometry (BD FACSCalibur System – 20,000 events for each test).
To examine statistical differences or similarities presented between the groups, we use the Wilcox on rank sums test. Groups were considered significative different for p values lower than 0.05 (for a two-tail hypothesis).
Zeta Potential Measurements
We built a special chamber for the measurements of zeta potential, consisting of two platinum electrodes (99.95%, Heraeus, São Paulo) separated by a channel of (length), (width) and (depth) (Fig. 1). After adding RBCs to the chamber, an individual RBC was trapped with the optical tweezers while an external electrical field was applied with a voltage power supply connected to the electrodes. Because the RBC is charged and the surrounding solution is electrolytic, the RBC will move at a constant terminal speed according to the applied voltage (V) (Fig. 2). In our method each RBC was submitted to different applied voltages (30, 40, 50, 60, 70 and 80 V) and the optical tweezers were used to recapture the cell after each voltage. Therefore, the terminal velocity (ν) was measured for each applied voltage for the same cell. A plot of the terminal velocity as a function of the electrical field E (E = V/d, where d is the distance between the electrodes) allowed us to obtain the zeta potential for each cell using the Smoluchowski equation:(1)where ε is the electrical permittivity of blood serum (1.06×10−9 C2/N m2) and η is the viscosity (1.65 cP) of the blood serum, measured with an Ostwald viscometer –. All data were recorded in real time and measurements of the terminal velocity were performed by video analysis with Image Pro-Plus software (Media Cybernetics, Silver Spring, MD) and Virtual Dub (by Avery Lee).
To evaluate the elasticity, RBCs were added to a Neubauer chamber, captured by the optical tweezers and dragged against the blood serum with six constant velocities ranging from 140 µm/s to 290 µm/s by using the motorized stage . When RBCs are dragged in blood serum they are deformed and two forces act upon the cells, a hydrodynamic force and an elastic force. Equilibrium occurs when elastic force cancels the drag force. At equilibrium,(2)where μ is the overall apparent elasticity ,  and ΔL = L−L0 is the cell length deformation (adopting L0 as the cell length in the absence of any force) η is the viscosity measured using an Ostwald viscometer and ν is the velocity. The cell is located at a distance Z1 from the bottom of a Neubauer chamber and Z2 from the cover slip, 1/Zeq = 1/Z1+1/Z2. Therefore, the measurement of the cell length as a function of the drag velocity can be used to extract a value for μ, once the plasma viscosity η, the initial length L0 and Zeq are known. The cell movement at six velocities was registered by the optical tweezers camera using a video capture card in a computer. The L value was extract from video images with Image Pro-Plus software (Media Cybernetics, Silver Spring, MD). The depth Z1 was measured by focusing the bottom of the Neubauer chamber and then lowering the chamber by the desired amount (in this case 50 µm) while keeping the cell fixed with the optical tweezers.
Figure 3 shows a representative plot of the velocity as a function of voltage for days 1, 8 and 29 used to obtain the zeta potential. Equation 1 shows that the slope increases with the zeta potential, therefore Figure 3 indicates that zeta potential decreases with the storage time. Table 1 shows the zeta potential measured during 36 days. On the first day of storage the ζ value was −14.5 mV, from the beginning of second week (day 8) up to the fourth week (day 22), it stabilized at an average value of −10.1 mV (p<0.001 when day 1 was compared to days 8, 15 and 22/for day 8: −9.7 mV, for day 15: −10.3 mV and for day 22: −10.2 mV), decreased to −8.5 mV during the fifth week (day 29) and remained practically constant after that (p<0.001 when day 1 was compared to days 29 and 36; p = 0.002 when the zeta potential of days 8, 15 and 22 was compared to days 29 and 36). The average zeta potential for RBCs in CPD-SAGM, therefore, decreased around 42% by the end of the storage period, compared to the cells on the first day of storage.
The higher the slope, the higher is the zeta potential. The correlation coefficients were better than 0.98. The barriers represent standard errors.
We also measured the RBCs zeta potential after leukoreduction to evaluate the influence of enzymes released from leucocytes lysed during storage. Table 2 shows that leukodepleted RBC samples presented an analogous zeta potential decay profile to the non leukodepleted ones (the small difference for day 1 is not significative; p = 0.4). This indicates that leukodepletion does not change RBCs membrane negative charges. Furthermore, these results also show that the presence of leukocytes enzymes is not responsible for the ζ decrease (p = 0.7 when days 8 and 15 for RBCs leukodepleted samples were compared to days 8 and 15 for RBCs non leukodepleted sample).
The decay of the ζ potential was anticorrelated with the production of ROS (in standard RBCs units). DCHF fluoresces only after oxidation and is proportional to the quantity of ROS produced. While the more pronounced ζ decay was during the first week of storage (30%), the percentage of cells presenting fluorescence raised by 60% in the first week indicating an increase of around 60% in the ROS production. The percentage of fluorescent cells increased only about 16% from day 8 until day 36. Thus, flow cytometry analysis suggests that the ζ decay is caused by membrane oxidative damages.
The loss of deformability can be easily observed by microscopic screenshots of trapped cells moving under crescent speeds in Figure 4 (this shows the difference between the elongation of cells of day 8 and of day 36 of storage). Figure 5 presents the result for RBC elasticity in CPD-SAGM as a function of the storage time. At the beginning of the second week of storage (day 8), the average elasticity value was (4.1×10−4±0.6×10−4) dyne/cm. This value presented no significant difference when compared to the elasticity (4.6×10−4±0.5×10−4) dyne/cm of day 22 (p = 0.5). On the fifth week, elasticity was (6.4×10−4±1.0×10−4) dyne/cm (p = 0.04 when compared to day 8), reaching the value of (9.6×10−4±1.0×10−4) dyne/cm at sixth week of storage (p<0.001 when compared to day 8 and p = 0.02 when compared to day 29). The apparent RBC elasticity has the same behavior of an elastic constant of a spring: the higher this value, the less elastic the RBC. Our results show that RBCs were 134% less deformable at the end of the storage period.
The ζ decay for non leukodepleted RBCs samples was more pronounced during the first week of storage (30%). The process of leukoreduction did not interfere in the electrical properties of the RBCs membrane and the ζ (of leukodepleted RBCs samples) presented similar decay. In other words, the results show that the ζ decay (and consequently the RBCs membrane charges decay) is not caused by the presence of leukocytes. These decays are consequence of general storage conditions related to, for example, the constituents of the preservative solutions, no matter if the RBCs came from leukodepleted samples. Furthermore, the analogous behavior observed in zeta potential and in ROS flow cytometry measurements indicates that the ζ decay is caused by oxidative damages generated by the production reactive species of oxygen.
Some reports suggested that young RBCs are more negatively charged than mature RBCs and that this could determine the mature cell sequestration in the reticulum-endothelial system –. Other authors reported that the removal of membrane sialic acid by sialidase enzymes (such as neuraminidase) in animal models decreased erythrocyte survival in vivo and the cells became more susceptible to rapid elimination from the circulation (cells were sequestered in the liver and spleen, probably due to greater adhesion among them – ). Moreover, endothelial cells are also negatively charged and electrostatic repulsion between RBCs and capillary walls can favor the blood flow through the microvasculature, suggesting that a loss of membrane charges can increase the adhesion of RBCs also to the capillary walls , . These findings reported from other authors support our idea that not only elasticity, but also RBC zeta potential is an important and sensitive property that undergo changes during the storage providing new insights for transfusion purposes.
In this study we observed that there is a gradual loss of elasticity during CPD-SAGM storage. We showed that elasticity remains practically preserved until day 22, suggesting that transfusion could be more effective (mainly in critical and special cases) when RBCs stored until this fourth week are used. The most significant loss of elasticity was observed during the fifth and sixth week of storage.
Band 3 protein is associated to the elastic behavior of RBCs. The sialylated protein most plentiful in RBCs membranes is the glycophorin A, consequently this protein is one of the main responsible for the RBCs negative membrane charges. Godin and co-authors reported that RBCs presented a loss of elasticity after treatment with neuraminidase and suggested that it was caused by a closer physical connection which exists between the glycophorin A and the Band 3 proteins . Therefore, the Godin and co-authors study show that changes in glycophorin A can reflect in changes in elasticity. Based on these evidences, the hypothesis of this paper is: the zeta potential decays induce a loss of cell elasticity that is observed more critically after the fourth week of storage as a consequence of oxidative damages during storage. Other supporting evidence for our hypothesis was the loss of RBCs deformability after gamma radiation observed by Brandão and co-authors  because it is well known that ionizing radiation can cause oxidative damages. These findings indicate that the preservation of the glycophorins integrity can be important to maintain RBCs elasticity.
In conclusion, in this study we used optical tweezers to quantify the elasticity and the zeta potential as function of storage time. We also pointed out how sensitive these two RBCs properties are to investigate RBC membrane injuries for clinical and research purposes.
The authors are grateful to Coordenação de Aperfeiçoamento de Pessoal de Nível Supeior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE), Fundação de Hematologia e Hemoterapia de Pernambuco (HEMOPE), L'óreal, Academia Brasileira de Ciências, Organização das Nações Unidas para a Educação, a Ciência e a Cultura (UNESCO) and Aggeu Magalhães Institute (FIOCRUZ – Fundação Oswaldo Cruz). This work is also linked to Optics and Photonics Research Center (CEPOF) from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), National Institute of Photonics Applied to Cell Biology, National Institute of Photonics and National Institute of Science and Blood Technology.
Conceived and designed the experiments: DCNS BSS CLC MLB-C AF. Performed the experiments: DCNS CNJ CALS. Analyzed the data: DCNS CNJ CALS HPF. Contributed reagents/materials/analysis tools: MMF AMDNC SCL. Wrote the paper: DCNS AF CLC.
- 1. Eylar EH, Madoff MA, Brody OV, Oncley JL (1962) The contribution of sialic acid to the surface charge of the erythrocyte. The Journal of Biological Chemistry 237: 1992–2000.
- 2. Pollack W, Reckel RP (1977) A reappraisal of the forces involved in hemagglutination. International Archives of Allergy & Applied Immunology 54: 2–42.
- 3. Jovtchev S, Djenev I, Stoeff S, Stoylov S (2000) Role of electrical and mechanical properties of red blood cells for their aggregation. Colloids and Surfaces A: Physicochemical and Engineering Aspects 164: 95–104.
- 4. Godin C, Caprani A (1997) Effect of blood storage on erythrocyte/wall interactions: implications for surface charge and rigidity. European Biophysics Journal 26: 175–182.
- 5. Durocher JR, Payne RC, Conrad ME (1975) Role of sialic acid in erythrocyte survival. Blood 45: 1–20.
- 6. Zubair AC (2010) Clinical impact of blood storage lesions. American Journal of Hematology 85: 117–122.
- 7. Fontes A, Barjas-Castro ML, Fernandes HP, Thomaz AA, Huruta RR, et al. (2011) Red blood cells mechanical and electrical properties using optical tweezers. Journal of Optics. A, Pure and Applied Optics 13: 044012.
- 8. Rookard LE, Edmondson O, Greenwell P (2009) ABO reverse grouping: effect of varying concentrations of the enzyme bromelain. Br J Biomed Sci 66(2): 93–7.
- 9. Parrow RL, Healey G, Patton KA, Veale KA (2006) Red blood cell age determines the impact of storage and leukocyte burden on cell adhesion molecules, glycophorin A and release of annexin V. Transfus Apher Science 34: 5–23.
- 10. Ashkin A, Dziedzic JM (1987) Optical Trapping and manipulation of viruses and bacteria. Science 235: 1517–1520.
- 11. Perkins TT (2009) Optical traps for single molecule biophysics: a primer. Laser & Photonics Reviews 3: 203–220.
- 12. Laliberte M, Bordeleau F, Marceau N, Sheng Y (2009) Antigen detection at atomolar concentration using optical tweezers. Proceedings of SPIE 7386: 738609.
- 13. Dinu CZ, Chakrabarty T, Lunsford E (2009) Optical manipulation of microtubules for directed biomolecule assembly. Soft Matter 5: 3818–3822.
- 14. Wang SK, Chiu JJ, Lee MR, Chou SC, Chen LJ, et al. (2006) Leukocyte–Endothelium Interaction: Measurement by Laser Tweezers Force Spectroscopy. Cardiovascular Engineering 6: 111–117.
- 15. Nascimento JM, Shi LZ, Meyers S, Gagneux P, Loskutoff NM, et al. (2008) The use of optical tweezers to study sperm competition and motility in primates. Journal of the Royal Society Interface 5: 297–302.
- 16. Pozzo LY, Fontes A, Thomaz AA, Santos BS, Farias PMA, et al. (2009) Studying taxis in real time using optical tweezers: Applications for Leishmania amazonensis parasites. Micron 40: 617–620.
- 17. Barjas-Castro ML, Brandão MM, Fontes A, Costa FF, Cesar CL, et al. (2002) Elastic properties of irradiated red blood cell units measured by optical tweezers. Transfusion 42: 1196–1199.
- 18. Brandão MM, Barjas-Castro ML, Fontes A, Cesar CL, Costa FF, et al. (2009) Impaired red cell deformability in iron deficient subjects. Clinical Hemorheology and Microcirculation 43: 217–221.
- 19. Brandão MM, Fontes A, Barjas-Castro ML, Barbosa LC, Costa FF, et al. (2003) Optical tweezers for measuring red blood cell elasticity: application to the study of drug response in sickle cell disease. European Journal of Haematology 70: 207–211.
- 20. Henon S, Lenormand G, Richert A, Gallet F (1999) A new determination of the shear modulus of the human erythrocyte membrane using optical tweezers. Biophysical Journal 76: 1145–1151.
- 21. Fontes A, Fernandes HP, Thomaz AA, Barbosa LC, Barjas-Castro ML, et al. (2008) Measuring electrical and mechanical properties of red blood cells with a double optical tweezers. Journal of Biomedical Optics 13: 014001.1–014001.6.
- 22. Mauritz JMA, Tiffert T, Seear R, Lautenschläger F, Esposito A, et al. (2010) Detection of Plasmodium falciparum-infected red blood cells by optical stretching. Journal of Biomedical Optics 15: 030517.1–030517.3.
- 23. Guck J, Ananthakrishnan R, Mahmood H, Moon TJ, Cunningham CC, et al. (2001) The Optical Stretcher: A Novel Laser Tool to Micromanipulate Cells. Biophysical Journal 81: 767–784.
- 24. Sze A, Erickson D, Ren L, Li D (2003) Zeta-potential measurements using Smoluchowski equation and slope of the current-time relationship in electroosmotic flow. Journal of Colloid Interface Science 261: 402–410.
- 25. Chelidze T (2002) Dielectric spectroscopy of blood. Journal of Non-Crystalline Solids 305: 285–294.
- 26. Hunter RJ (1981) Zeta potential in colloid science: principles and applications. London Academic Press.
- 27. Zhu C, Bao G, Wang N (2000) Cell mechanics: Mechanical responses, cell adhesion, and molecular deformation. Annual Review of Biomedical Engineering 2: 189–226.
- 28. Raat NJ, Verhoeven AJ, Mik EG (2005) The effect of storage time of human red cells on intestinal microcirculatory oxygenation in a rat isovolemic exchange model. Critical Care Medicine 33: 39–45.
- 29. Danon D, Marikovsky Y (1961) Difference de charge eletrique de surface entre erythrocytes jeunes et ages. Comptes Rendus de l'Académie des Sciences 253: 1271–1272.
- 30. Yaari A (1969) Mobility of Human Red Blood Cells of Different Age Groups in an Electric Field. Blood 33: 159–163.
- 31. Aminoff D, William F, Vorder B, William CB, Keith S, et al. (1977) Role of sialic acid in survival of erythrocytes in the circulation: Interaction of neuraminidase-treated and untreated erythrocytes with spleen and liver at the cellular level. Proceedings of the National Academy of Sciences of the United States of America 74: 1521–1524.
- 32. Danon D, Skutelsky E (1976) Endothelial surface charge and its possible relationship to thrombogenesis. Annals of the New York Academy of Sciences 275: 47–63.
- 33. Born GVR, Palinski W (1989) Increased microvascular resistance to blood flow in the rat hinglimb after perfusion with neuraminidase. The Journal of Physiology 419: 169–176.